Rotor-equipped mixing devices such as screw-type extruders and batch mixers are well known and are described in considerable detail in Mixing and Compounding of Polymers, Theory and Practice, I. Manas-Zloczower et al, editors; Hanser/Gardner Publications, Cincinatti, Ohio, 1994 and in Extrusion of Plastics, Second Edition, by E. G. Fisher; John Wiley and Sons, London, 1964. Included therein are guidelines for choosing among various extruder screw configurations, including co-rotating and counter-rotating, meshed and non-meshed, self-wiping and non-self-wiping screws. Rotors provided in batch mixers vary considerably in design from simple paddles to complex geometries.
Typically, screw-type extruders are employed to extrude a formable material through a flow-restrictive orifice to form an extrudate of controlled cross-section. Such a machine is unsuitable for conveying and discharging a non-formable material such as a granular material. However, an extruder can be operated to convey and discharge a non-formable granular solid, by simply removing the flow-restrictive orifice or die at the extruder exit.
Machines with rotors and paddles especially designed for blending and conveying granular materials are known in the art. These machines in general are not equipped with screws capable of generating significant thrust and are thus not suitable for processing molten thermoplastics. See Mixing, Vol II, by V. Uhl and J. Gray, p. 280, Academic Press, New York, 1966.
Machines for melting, mixing, devolatilizing, and pressurizing polymers and other substances equipped with a multiplicity of rotors are preferred for mixing. Typical such machines are screw extruders, multiple-screw or twin-screw extruders being preferred over single-screw extruders for conveying materials while they are being mixed. Fully meshed, co-wiped co-rotating screws are generally preferred for mixing operations.
In a usual continuous process, the residence time of materials in an extruder is very short. It, therefore, often is difficult to achieve thorough mixing in one pass. Further, if the feed throat of an extruder is open to the atmosphere, it is difficult to achieve or maintain a pressurized condition in an extruder when processing granular solids, although this can be more readily accomplished when processing molten plastics.
In conventional multiple-rotor batch mixers, the materials to be processed are introduced into a chamber typically containing two rotatable mixing tools. Mixing can be continued for any desired period; that is, the residence time is in principle unlimited. At the conclusion of mixing, the chamber is opened, and the material is removed from the mixing chamber, typically by means of a combination of gravity and scraping or by using a discharge screw. A batch mixer can be operated at any desired gas pressure, including gas pressures that are in general much higher than those obtainable in an extruder.
Extruders thus have the advantage of rapid and continuous processing, along with ease of material handling, while batch mixers have the advantage of unlimited mixing times and higher gas pressures.
Scheuring, U.S. Pat. No. 5,332,308, combines a batch mixer with an extruder into a single integrated apparatus consisting of a batch mixer situated above one end of a twin-screw extruder the direction of rotation of the screw of which is reversible. In operation, the screws initially rotate so as to convey ingredients toward and retain them in the batch mixing unit until the desired degree of mixing has been achieved, at which point the direction of rotation of the screws is reversed and the material is conveyed to the exit die of the extruder and thence extruded.
Osrow, U.S. Pat. No. 4,391,575, discloses a kitchen device for making dough-like products that combines a single-screw extruder with a mixer into a single, integrated device. The screw rotates to convey material to the batch mixer during mixing; the direction of screw rotation is reversed at the conclusion of mixing in order to discharge the material.
Inoue, Japanese Patent Application SHO 63(1988)-32472, discloses a combination of an extruder and feed system which operates by metering into the extruder screw a predetermined quantity of material to be mixed as the screw turns in the forward direction. After the predetermined amount is admitted, the feeder is closed, and the direction of the screw is reversed. The screw direction is then cycled forward and reverse at predetermined intervals to achieve mixing. Finally, the screw is rotated in the forward direction to extrude the mixed ingredients through a breaker plate into a cavity for further processing.
Inoue's device does not address operations under pressure. Further, the employment of a breaker plate at the exit of the extruder does not permit processing granular solids. Finally, the open extruder barrel is undesirable for processing molten materials because of the potential for leakage.